The invention is based on a vehicle brake system as defined hereinafter.
German patent applications DE-OS 24 33 092, DE-OS 24 49 481, DE-OS 24 59 775 and DE-OS 37 25 811 all disclose vehicle brake systems having dual-circuit brake pressure sources that are controllable with a brake pedal, each having two brake circuits terminating at wheel brakes and an anti-skid apparatus, which has a brake pressure modulator built into one of the brake circuits and a brake pressure adaptor built into the other brake circuit. The brake pressure adaptors each comprise a cylinder, a piston, a restoring spring, and a 2/2-way valve that is closable by the piston when it is displaced counter to the restoring spring. The applicable cylinder communicates with the brake pressure modulator via a control connection and is controllable by the brake pressure modulator. A front wheel brake and a diagonally opposed rear wheel brake are connected directly to a brake pressure modulator, for instance in accordance with DE-OS 24 33 092. Via the associated brake pressure adaptor, the other front wheel brake and the other rear wheel brake can be supplied with brake pressure from the brake pressure source. In this brake system, the brake pressure modulator is controlled as a function of the rotational behavior of at least one front wheel, which can be driven in common with the other front wheel via a differential gear and the rotation of which can be observed by means of a wheel rotation sensor disposed on the differential gear. Brake pressure modulators therefore operate by the select-low principle with respect to the two front wheels, so that a danger of locking at one front wheel leads to reductions in brake pressure at all the wheel brakes.
In the vehicle brake system of DE-OS 24 59 775, both front wheel brakes are jointly connected to the brake pressure modulator, and the brake pressures of the rear wheel brakes are jointly adaptable to the front wheel brake pressures by means of the brake pressure adaptor. Each front wheel has its own wheel rotation sensor. The controlling intervention via a control unit upon the single brake pressure modulator of this vehicle brake system is effected either by the select-low principle or by intermittent alternation between the select-low and the select-high principle. The anti-skid apparatus operating by the select-low principle has the disadvantage that the wheel rotation sensor of which the signals first show a danger of wheel locking can reduce the brake pressure in the entire brake system. This has the disadvantage of long stopping distances. Although the anti-skid apparatus that is operated by the select-low and the select-high principle in alternation does intermittently avoid disadvantageously extensive brake pressure reduction, it requires complicated control logic, which still cannot always prevent locking of at least one wheel. Since in addition to the two front axle wheel rotation sensors, a wheel rotation sensor is also disposed on the differential gear of the rear axle, a detected danger of the locking of a rear wheel, tripped for instance by cornering, in the select-low mode can lead to a substantial brake pressure reduction. On the other hand, if the brake pressure modulator is controlled in the select-high mode, then depending on prevailing conditions the vehicle may unexpectedly tend to oversteer or understeer when cornering.
These previously known vehicle brake systems include safety devices, which assure that if the controllable brake pressure modulators should fail, the pressure of the brake pressure source in the other, still-intact brake circuit reaches the wheel brakes associated with the brake pressure adaptor unhindered, thus assuring the regulation minimum stopping times of the vehicle. The safety devices for instance comprise 3/2-way valves of complicated structure, which are hydraulically controllable by means of pressures of the brake pressure source.
U.S. Pat. No. 4,491,919 and European Patent 0 178 817 disclose vehicle brake systems equipped with two brake circuits and wheel brakes for four wheels, the anti-skid apparatuses of which each have two individually controllable brake pressure modulators. The left front wheel has one of the brake pressure modulators assigned to it, which as a function of the rotation of this front wheel modulates the brake pressure of its front wheel brake and at the same time the brake pressure of a diagonally opposed rear wheel brake. The other brake pressure modulator is similarly assigned to the other front wheel and also modulates the brake pressure of the diagonally opposed rear wheel brake. In cornering, for instance with high sideways acceleration, the brake pressure of the front wheel on the inside of the curve is reduced earlier and/or to a greater extent than that of the front wheel on the outside of the curve. This has the disadvantage that the rear wheel on the outside of the curve contributes little to the vehicle braking, and that the rear wheel on the inside of the curve tends toward skidding. Because much less slip occurs on the rear wheel on the outside of the curve than on the front wheel on the outside of the curve, the vehicle may unexpectedly, and disadvantageously, oversteer when the anti-skid mode begins. For instance, if the wheels on one side of the vehicle are traveling on a strip of road with good traction, and the wheels on the other side of the vehicle come into contact with wet, slippery ice, then the brake pressure modulator of the front wheel that is moving on the road with good traction causes the rear wheel diagonally away from it, traveling on the ice, to lock or skid. For the front wheel running on the ice, the brake pressure is reduced virtually to zero, with the disadvantageous result that the rear wheel diagonally opposite it contributes practically nothing to vehicle braking. Under these conditions, even though two independently functioning brake pressure modulators have been installed, the stopping distances are therefore disadvantageously long.
Another vehicle brake system with four brakable wheels, known from U.S. Pat. No. 4,600,244, has an anti-skid apparatus with two electrically controlled brake pressure modulators, two wheel rotation sensors, and two brake pressure adaptors, each of which again have a cylinder, a piston with a restoring spring, and a 2/2-way valve that is closable as a function of the piston motion. A brake pressure modulator and its wheel rotation sensor are assigned to one front wheel and its wheel brake. The other brake pressure modulator and its wheel rotation sensor belong to a diagonally opposed rear wheel and its wheel brake. The brake pressure adaptor connected to the brake pressure modulator of the front wheel is intended for the opposed front wheel brake. Logically, the other brake pressure adaptor connected to the brake pressure modulator of the rear wheel brake is intended for the second rear wheel brake. It will be appreciated that brake pressures are accordingly modulated per axle. On the one hand, this may have the advantage that despite load-dependent and deceleration-dependent changes in axle load during decelerated straight-ahead travel, good utilization of the traction of a road by the wheels of the front axle and the wheels of the rear axle is attained; but on the other hand, while cornering, wheels on the inside of the curve may be overbraked, or wheels on the outside of the curve may not be braked as hard as possible, depending on whether the wheel rotation sensor associated with one vehicle axle happens to be on the side of the vehicle that is on the outside or the inside of the curve. Experience has shown that on roads meant for traffic that normally keeps to the right, the strip of road used by the right wheels of a vehicle often freezes sooner than the strip used by the left wheels of the vehicle. For the association of the front wheel rotation sensor with the right front wheel disclosed in this known vehicle brake system, this has the disadvantage that if the rightward strip of the road freezes, the left front wheel, which typically is still on a strip of road that has traction, is virtually unbraked, thus resulting in longer stopping distances. If this happens when the vehicle is cornering to the left, relief of the rear wheel on the inside of the curve can make the braking forces at this rear wheel virtually disappear.